# Another how to draw a parallel line that pass a specific point question

5 views (last 30 days)
as hz on 28 Oct 2013
Commented: Image Analyst on 29 Oct 2013
Hi,
I cannot find what is the problem with my code as the output is not what I want. This is a follow up question to http://www.mathworks.com/matlabcentral/answers/104062-draw-a-parallel-line-thst-pass-a-specific-point
My aim is to generate a parallel line to another line that pass through a point. At the beginning I choose two points where a line is passed via them, then I choose a third point and a parallel line that passes through it is created. But the parallel line slope is not correct. The parallel line start and end X points are the beginning and end of the image dimensions.
I have attached the output which visually shows the problem. Therefore, what is the problem and why it does not work? (I now its a simple geometry and that’s what frustrate me).
Thanks a lot.
Code:
clc;
clear;
figure, imshow(I);
[ximage yimage] = size(I)
b1 = impoint(gca,[]);
b2 = impoint(gca,[]);
b3 = impoint(gca,[]);
pos1 = getPosition(b1);
x1=pos1(1,1);
y1=pos1(1,2);
pos2 = getPosition(b2);
x2=pos2(1,1);
y2=pos2(1,2);
pos3 = getPosition(b3);
x3=pos3(1,1);
y3=pos3(1,2);
slope = ((y1-y2)/(x2-x1))
hold on
plot([x1,x2],[y1,y2],'Color','r','LineWidth',2);
x4=0;
x5=ximage;
y4 = slope * (x4 - x3) + y3
y5 = slope * (x5 - x3) + y3
impoint(gca,x4,y4);
impoint(gca,x5,y5);
plot([x4,x5 ],[y4,y5],'Color','r','LineWidth',2);

Image Analyst on 28 Oct 2013
Your slope is wrong, plus I made numerous other improvements. See this:
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
imtool close all; % Close all imtool figures if you have the Image Processing Toolbox.
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
imshow(grayImage);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off')
[rows, columns] = size(grayImage)
message = sprintf('Click and drag the endpoints of the line\nand double-click the double arrows to finish.');
uiwait(helpdlg(message));
h = imline;
lineEndPoints = wait(h)
delete h;
x1 = lineEndPoints(1,1);
y1 = lineEndPoints(1,2);
x2 = lineEndPoints(2,1);
y2 = lineEndPoints(2,2);
hold on
plot([x1,x2],[y1,y2], 'r-', 'LineWidth',2); % Make line.
plot([x1,x2],[y1,y2], 'yo', 'LineWidth',2,'MarkerSize', 12); % Make circles.
message = sprintf('Click a third point.');
uiwait(helpdlg(message));
[x3, y3] = ginput(1);
plot(x3, y3,'yo','MarkerSize', 12);
% Calculate the slope.
slope = (y2 - y1)/ (x2 - x1)
% Draw second line.
x4=0;
x5=rows;
y4 = slope * (x4 - x3) + y3
y5 = slope * (x5 - x3) + y3
plot([x4,x5 ],[y4,y5],'r-','LineWidth', 2);
as hz on 29 Oct 2013
Thanks. I saw the error.
what will you adivse to do if I draw a vertical line which results with no slope (infinity)?
As always thanks.
Image Analyst on 29 Oct 2013
That needs to be detected in advance and treated as a special case.

### Categories

Find more on Large Files and Big Data in Help Center and File Exchange

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!